I have a 1950 GMC 2.5 ton Dump Truck which starts and runs fine but loses oil pressure after it warms up.?

Question

I have installed a new oil pump, replaced the main and rod bearings, removed the hose to the external oil filter and plugged the hole in the side of the block where this hose connected to the oil passage and replaced the oil with brand new oil (SAE 30 WD), but none of these actions made any difference. Any suggestions what to try next?

Answer 1

I own a shop, and have built many engines in my day. It sounds like your lifter bores are worn too much, and when the engine gets hot they expand even more, thus creating a loss of pressure at the top of the engine. I would try hooking the oil gage to different places on the block to see if its maintaining pressure at these points. It depends on where the oil sending unit is hooked up at, and can you depend on the gage? There could even be trash in the line if you have a gage. I suspect the gage is hooked at about the end of the line for the oil galley. What about hooking it to the place you unhooked the hose for the external filter? I would try going to a 20w 50w oil, and see if this gets it up there. Also; are the drain back holes in the block stopped up? Maybe the oil is pumping to the upper part of the engine, and staying up there so long that it is starving the pump?
Glad to help out, Good Luck!!!

Answer 2

You must have a newer engine than what came in your truck since you said you changed the rod bearings. Or else it has been converted to have replaceable inserts, I have seen a few of those. Those early engines like that originally had poured babbit rod bearings that were not replaceable.
Not knowing exactly which type of engine you may have I'd say to suspect the cam bearings next . Or if your engine is the original block it may have a broken "sprayer" line . Or maybe the block has a missing or otherwise oil gallery plug. Last of all , if this is the original engine type, most of those I've ever seen never had more than10 psi or so at idle when warm. This was due to the fact that the oil pump only supplied oil to the mains.

Answer 3

Make sure the oil pressure dash gauge or sending unit are not bad by hooking up a mechanical gauge to the oil pressure sending unit hole and check the pressure. I'm guessing a truck that old has a bad/inaccurate sender or dash gauge. Normal pressure at idle is usually around 20-30 PSI and around 60 at higher RPM's. Could also be losing pressure at the lifters or cam bearings (worn) and possibly a bad/leaking pickup tube and screen if it's still actually low. Check pressure at the filter boss to see if it's ok there which would indicate a pressure loss in the top end.

Answer 4

the 1st concern is to examine the sending unit to make certain if the gauge is analyzing good. Use a stress gauge linked to the port that the sending unit screws into. whilst the engine is revved up prevalent stress could be approximately 60 psi. If there is not any longer something incorrect with the gauge or sending unit unit the priority could be a wiped out oil pump which isn't a important fix, in step with danger $200-$3 hundred. If no longer the pump you probable have an inner engine concern and are observing a quite a few thousand dollar fix.